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The classical planets (Mercury, Venus, the Sun, Mars, Jupiter and Saturn) were considered to be "wandering stars" - and the word "planet" means just that.

Wikipedia says "By the 17th century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers."

Since the planets were considered stars in those times, the question is when did the astronomers realize that they are something else?

The same question in put in another words: When did the scientists first hypothesize that the planets do not emit their own light, like stars, and they just reflect the star from the sun? And when was that proven for the first time?

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    $\begingroup$ This may be better on the History of Science stack exchange. $\endgroup$ – James K Aug 11 '17 at 7:56
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On p.33 of the "Cambridge Concise History of Astronomy" it states that Hellenistic astronomers interpreted the variation in the brightness of the planets as a sign that they were closer or further away at various points of time. Unfortunately it doesn't have a deeper discussion here.

But on p.45 it discusses Ptolemy's thinking - he thought Mercury was, at its closest point, as close as the Moon and that the fixed stars were at the same distance as Saturn at its furthest point - the latter point suggesting that while the planets were seen as different, they might not have been thought of as being that different beyond the way they wandered.

To back that up further there is, for instance, a copy of an image from the Nuremberg Chronicle of 1493 on p.77 - here the planets are merely shown as wandering stars.

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The moon has been known to be lit by the sun since antiquity. Since the moon was known to be lit by the sun, it is not much of a leap to believe the other planets were too. The Pythagoreans believed all the Planets (including the sun and fixed stars) were lit by the light of the "central fire". So the notion that the planets reflected light is ancient. This notion is consistent with Ptolemy.

As for proof, at least since Galileo in about 1610. Galileo observed the phases of Venus, which he interpreted as a spherical body being lit by the sun from various angles.

It is notable that this does not appear to have been a "wow" moment. It confirmed what Galileo already believed about the orbit of Venus, he does not appear to be have been greatly surprised that the planets show as discs in a telescope. I surmise that the fact that planets reflect the light of the sun was generally believed by Muslim and Western astronomers since ancient time.

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I don't know when it was first hypothesised that the planets were different from the stars. Given that stars twinkle, and planets don't (at least not so much), it could well have been in classical times. However the first proof would have come with the use of telescopes to look at the sky, so around about Galileo's time. This would have given proof because the planets would resolve into disks, and the bands of Jupiter were noted almost immediately. Stars however remained pointlike irrespective of magnification applied. Also telescopes would have shown that that Mercury & Venus, and to a lesser extent Mars exhibit phases, like the moon.

There is a further point as to when it was realised that the stars and the sun were the same type of object. Again I don't know when this was first hypothesised, but fairly compelling evidence would have come with the advent of stellar spectrometry, which I think was was mid 19th Century. This is because the stars would show similar patterns of absorption lines as the sun.

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  • $\begingroup$ Nice answer. You skipped an important step with Friedrich Bessel in 1838 solar-center.stanford.edu/FAQ/Qsunasstar.html who was the first to measure the distance of a star by parallax. With distance they could calculate brightness and that gave scientists pretty good certainty that stars were similar to our sun and very different than planets. Astronomical Spectroscopy came closer to the end of the 19th century when methods were improved. en.wikipedia.org/wiki/Astronomical_spectroscopy $\endgroup$ – userLTK Aug 11 '17 at 9:26
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    $\begingroup$ @userLTK good point re Bessel and parallax, although I suspect the main surprise was just how much much further away the "fixed" stars were - Bessel got pretty close at about 10 light years for 61 Cygni. $\endgroup$ – Dr Chuck Aug 11 '17 at 20:19

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